This invention relates to microwave electron tubes and more particularly to certain modifications to the slow wave structure of a crossed-field amplifier (CFA) known as a DEMATRON. This name is an acronym for the term "distributed-emission magnetron amplifier." The device includes a linear-format, ladder-type slow wave structure including a pair of oppositely wound helices and a pair of trimmers on either side of the ladder array. The slow wave structure extends along one side of the evacuated tube envelope and a cold cathode extends along the other side thereof, with a collector electrode near the output end of the device. The space between the slow wave structure and the cathode comprises the interaction space, in which energy exchange takes place between the electron beam and the rf field propagating down the slow wave structure, provided the phase velocity of the rf wave is about the same as the electron beam velocity. The DEMATRON differs from most devices in that it is rf keyed, which means that operation of the tube is initiated by the application of rf input at the cathode end of the slow wave structure, and the operation ceases again upon removal of the input. Since the cathode is cold, the standby power is zero, and no control electrode is necessary.
The invention comprises a modification of the aforementioned trimmer dimensions in the vicinity of the cathode of the device. This modification was found to increase the tube gain.